The present invention relates to a piston pump containing a delivery piston, a pressure-limiting piston, a restoring spring, a pressure-controlling spring and two non return valves, which is preferably suitable for ensuring a sufficient hydraulic pressure for actuating a variable valve timing gear in internal combustion engines.
A variable valve timing gear for internal combustion engines is disclosed in EP 1 046 793. In this case, the stream of lubricant produced by the existing lubricant pump is used to actuate an adjusting device situated between the impeller and camshaft at the end of the camshaft, said adjusting device in turn causing a phase displacement of the camshaft. The abovementioned adjusting devices are supplied as a rule through holes in the camshaft, the stream of lubricant generally being controlled by solenoid valves.
However, at low engine speeds, even when the quantity of lubricant is sufficient, the pressure of the lubricant may be too low to actuate the adjusting devices, or else the actuation takes place too slowly. Typically, a lubricant pump having increased delivery capacity has been installed in order to compensate for this known drawback. However, at relatively engine high speeds the pump consumes a lot of power, which is undesirable, since it reduces engine efficiency. As an alternative, two-stage pumps or variable pumps are used which ensure a sufficient lubricant pressure at low speeds, but do not have such high power consumption at high speeds. However, these pumps are very cost-intensive.
A pump of the type mentioned at the beginning is disclosed in EP 0 976 926. It is used to deliver a fluid medium, for example fuel, and/or to increase the pressure. For this purpose, a piston which can move up and down in a cylindrical hole is driven by a cam directly or by means of a tappet push rod. During the upward movement of the piston the fluid flows through the outlet passage, while at the same time fluid passes onto the second side of the piston through a non-return valve. The downward movement of the piston allows the fluid to flow, while the first-mentioned non-return valve closes, through a second non-return valve into the cylinder.
The inventors herein have recognized that this type of pump also reaches very high liquid pressures, and therefore reduces engine efficiency.
Against this background, the present invention teaches a pressure-increasing pump that ensures an approximately uniform liquid pressure and at the same time avoids the disadvantages of the prior art.
In carrying out the features and advantages of the present invention, pump comprising a housing; a transfer piston slidably positioned in said housing; a pressure-controlling piston slidably positioned in said transfer piston; and an energy-accumulating device controlling movement of said pressure-controlling piston within said transfer piston, said movement of said pressure-controlling piston providing an essentially uniform fluid pressure within said housing is presented.
The cylinder head of an internal combustion engine contains at least one camshaft, which is provided with a known variable valve timing device. In the advantageous embodiment shown here, the cylinder head contains a housing in which the individual parts of the pressure-increasing pump are accommodated. This housing can be an integral part of the cylinder head or can be appropriately fitted as a separate part. Situated in the housing is a transfer piston which is driven in a known manner, directly or by means of a tappet, by a cam, already present, of the engine valve timing gear, or, advantageously, by an additional cam situated on the camshaft. In an advantageous embodiment, the transfer piston is designed as a hollow part having a fluid-outlet and pressure-equalizing hole lying transversely at the cam-side end. The transfer piston is closed on the cam side and bears against the camshaft, while the opposite end is open. The open end of the transfer piston is closed by the pressure-controlling piston, which, for its part, is pressed by the pressure-controlling spring against a snap ring that is embedded in a groove in the transfer piston. Further components of the pump also include a compression spring, which guides the transfer piston frictionally on the cam in a known manner, and one to two non-return valves. In this case, the non-return valve, which, in an advantageous embodiment is held by the compression spring, opens during the intake stroke to the inlet passage, integrated into the housing, while the second non-return valve, which is fitted laterally here, releases liquid into the delivery passage when the desired delivery pressure is reached.
An advantage of the present invention resides in the fact that all the parts belonging to the pressure-increasing pump are pre-assembled in an insert, so that simple assembly of the pump is possible.
The above advantages and other advantages, objects and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.